1. Field of the Invention
The present invention relates to a four-stroke-cycle engine provided with a continuously variable transmission mechanism driven by means of belt and particularly applicable to a snowmobile.
2. Related Art
A four-stroke-cycle engine provided with a continuously variable transmission mechanism driven by means of belt (hereinafter referred to as a belt driven-type CVT mechanism or merely CVT mechanism) includes a crank shaft having one end projecting outward from the engine, and an input side of the CVT mechanism is connected to this projecting end of the crank shaft (hereinafter which may be referred to as a PTO (power take-off) shaft) such as shown in Japanese Patent Laid-Open (KOKAI) Publication No. HEI 3-208784 and Japanese Patent Laid-Open (KOKAI) Publication No. HEI 11-153035.
Because tension of a belt of the CVT mechanism is applied to the PTO shaft, it is preferable to set the PTO shaft to be shorter in length. Especially, in order to ensure sufficient strength, it is required to increase a distance between the PTO shaft and a bearing therefor for increasing the diameter of the crank shaft including the PTO shaft. In addition, it is also required to use a large-sized bearing, which leads to upsizing of the whole structure of the engine and increased weight thereof.
In addition, in order to decrease the length of the PTO shaft, it is preferable to take out a power for driving a cam chain or auxiliary equipment from the other component than the PTO shaft. In case where the engine is provided with a balancer to attenuate vibration of the engine, a power for driving the balancer is drawn normally from the crank shaft. Even in such a case, it is also preferable to take out the power from the other component than the PTO shaft.
The crank case is generally composed of a plurality of crank case components which are mated, in assembling the crank case, through respective mating surfaces so that a balancer shaft of the balancer is rotatably supported in the crank case between the mating surfaces of the crank case components, as like as for the crank shaft, thus improving assembling properties and providing advantageous effects in cost.
However, a magneto device is normally placed at one end of the crank shaft, which is opposite to the PTO shaft, thus making it difficult to provide, on such a side of the crank shaft, a power take-off unit for a cam chain or auxiliary equipment, or another power take-off unit for the balancer.
In the structure of the prior art Japanese Patent Publications mentioned above, tension (tensioning) direction applied to a belt member for the CVT mechanism accords or may accord with the direction of the mating surfaces of the crank case components. In such arrangement, when a metallic bearing is for example used and the tension direction of the belt of the CVT mechanism is consistent with the direction the mating surfaces of the crank case components, the a service life of the bearing may be deteriorated and shortened, thus providing unfavorable results.